test(theming): Add unit test to check database query in theming backgroundjob

[nickvergessen]

I observed the following error:

Error while running background job OCA\Theming\Jobs\RestoreBackgroundImageColor

An exception occurred while executing a query:
SQLSTATE[42601]: Syntax error: 7 ERROR: syntax error at or near
"" FROM "" LINE 1: ..."oc_preferences" "a" LEFT JOIN "oc_(SELECT "userid" FROM "oc...
^

Now trying to see if it's only postgres

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[nickvergessen]

32+ migration is still broken on Oracle
cc @sorbaugh please take over with your team